Hitherto, semiconductors have been continuously improved in performance, costs and power consumption by reduction of a gate length and a gate thickness of transistors, i.e., so-called micronization thereof. However, if it is intended to achieve the micronization of transistors as recently required, a gate thickness of a conventional gate insulating film using silicon oxide becomes excessively small, so that a leakage current owing to a tunnel current increases, and power consumption becomes large. In addition, in recent years, there is an increasing demand for mobile equipments using semiconductor devices such as mobile phones, notebook type personal computers and potable music players. In this case, a power supply for these mobile equipments has been frequently relied upon rechargeable batteries. Therefore, it has been required that the semiconductor devices used in the mobile equipments have a low power consumption to realize long-term use thereof. In consequence, for the purpose of reducing a leakage current during a stand-by state of the equipments, there has been proposed the technique in which as combination of an insulating material and a gate electrode as constituents of a transistor, combination of a high dielectric material and a metal gate is used in place of the conventional combination of silicone oxide and polysilicon. In the conventional technique, aluminum is selectively used as one of metals for the metal gate (Patent Document 1).
There have been conventionally proposed various methods for producing the high dielectric material and the metal gate. As one of the conventional methods, there has been proposed a so-called gate-last method in which after producing a transistor from combination of a high dielectric material and a polysilicon, the polysilicon is removed to replace it with a metal gate (Non-Patent Document 1). In FIG. 1, there is shown a schematic sectional view of a part of a transistor before removing a polysilicon from a semiconductor device using a high dielectric material. In order to remove the polysilicon by etching, there may be used an alkaline solution. The polysilicon is usually exposed to an atmospheric air before subjected to the etching, so that a natural silicon oxide film is produced on a surface of the polysilicon. However, it is not possible to remove the natural silicon oxide film thus produced using the alkaline solution. In addition, around the polysilicon, there are present aluminum, an interlayer insulating film and a side wall as portions which should not be removed by the etching. For these reasons, there is a demand for techniques of removing a natural silicon oxide film without removal of aluminum, an interlayer insulating film and a side wall before etching the polysilicon with an alkaline solution.
Successively, it is necessary to etch the polysilicon thus exposed. Further, the high dielectric material exposed by etching the polysilicon is also the portion which must not be removed by etching. For these reasons, it is required to provide a technique of etching the polysilicon exposed by removing the natural silicon oxide film without removing the aluminum, interlayer insulating film, side wall and high dielectric material by the etching.
As a technique of removing a natural silicon oxide film by etching, there has been proposed the technique in which a natural oxide film of a polysilicon is removed with diluted hydrofluoric acid, and then a uniform chemical oxide film is formed thereon (Patent Document 2). However, in this technique, aluminum is also etched. Therefore, the technique is not applicable to the above selective etching procedure (refer to Comparative Examples 1-1 and 2-1).
As a technique of selectively etching a natural silicon oxide film and a thermal oxide film, there has been proposed the technique in which a natural silicon oxide film on a substrate is removed by introducing a mixture of a vapor of a hydrofluoric acid aqueous solution and a vapor of a diluting solvent such as 2-propanol into a reaction chamber evacuated to a vacuum pressure (Patent Document 3). However, since aluminum is etched by hydrogen fluoride, the above technique has failed to obtain the semiconductor as desired. In addition, in the above technique, the natural silicon oxide film and the thermal oxide film are respectively etched by utilizing the difference in etching initiation time therebetween. More specifically, the above technique substantially relates to a method of etching the thermal oxide film. Therefore, when using the above technique, it is required to very strictly control an etching treatment time of the respective oxide films, which will result in difficulty in use of the technique.
In addition, as a technique of selectively etching a natural silicon oxide film and a thermal oxide film, there has also been proposed the technique in which the oxide films are subjected to heat treatment at a temperature of from 900 to 1050° C. in a hydrogen atmosphere to remove the natural silicon oxide film (Patent Document 4). One feature of the method of removing a polysilicon to replace the polysilicon with a metal gate resides in that the heat treatment is carried out while controlling an impurity region of source and drain with a high accuracy, so that an additional heat treatment is no longer required, and diffusion of impurities can be caused as designed. However, in this technique, when the natural silicon oxide film is removed, unexpected diffusion of impurities tends to occur owing to the heat treatment, which will result in failure to attain properties as desired.
As a technique of etching, a natural silicon oxide film, there has been proposed the dry-etching technique using a fluorine-based gas (Patent Document 5). However, upon the dry-etching process using a fluorine-based gas, an interlayer insulating film also tends to be undesirably etched, which will result in failure to obtain a semiconductor as desired.
In consequence, it is required to provide a process for producing a transistor including a laminate formed by laminating at least a high dielectric material film and an aluminum metal gate in which a natural silicon oxide film is selectively removed by etching without etching aluminum, an interlayer insulating film and a side wall.
As a method of etching a polysilicon, there is also known the method of subjecting a polysilicon to dry etching (Patent Document 1). However, since aluminum and an interlayer insulating film are also etched in the dry etching process, it is necessary to provide a protective film such as a photoresist on the aluminum and interlayer insulating film. If such a protective film is formed, the production process tends to become complicated, so that there tend to arise the problems such as poor yield and increase in production costs. In addition, an ashing treatment required to remove the photoresist tends to cause damage to the aluminum and interlayer insulating film, which tends to cause a risk of deteriorating a performance of transistors. Also, in general, for the purpose of preventing generation of fine silicon residues, the silicon is subjected to so-called overetching in which the etching is carried out for a longer period of time than an etching treatment time calculated from an etching amount of silicon per unit time (hereinafter referred to as an “etch rate”). In the dry-etching process, a high dielectric material exposed after etching the silicon tends to be etched or tends to be deteriorated in quality when subjected to the overetching, resulting in deterioration in performance of transistors.
As a cleaning solution used upon etching silicon by a wet etching method, there are known various alkaline cleaning solutions (Non-Patent Document 2). However, these cleaning solutions tend to etch not only the polysilicon but also aluminum (refer to Comparative Examples 2-6).
As a technique of etching silicon without etching aluminum, there has been proposed the etching solution for anisotropic etching of silicon which is prepared by dissolving silicon in tetramethyl ammonium hydroxide (Patent Document 6). However, in this technique, the etching solution must be used at an elevated temperature. Therefore, when the above technique is carried out using a sheet cleaning apparatus for cleaning a silicon wafer one by one which has been recently used usually in production of semiconductors to suppress generation of particles in a wet etching method, it is not possible to attain a stable etching capability. If the etching is conducted at a temperature at which the sheet cleaning apparatus is usable, the etch rate of silicon tends to be excessively low. Thus, the above technique is not applicable to etching of silicon in a step of forming a transistor including a high dielectric material and a metal gate. In addition, in the above technique, precipitates are generated at a reduced temperature. For this reason, the technique is not applicable to a step of forming a transistor portion of semiconductors in which even fine particle residues are not allowed to remain (refer to Comparative Example 2-7).
As an etching agent composition capable of selectively etching silicon only by anisotropic etching thereof without etching aluminum or aluminum alloys, there has been proposed the alkali-based etching agent composition prepared by adding a reducing compound and an anticorrosive agent to an alkali aqueous solution (Patent Document 7). However, in this technique, since an etch rate of aluminum is excessively high, the technique is not applicable to etching of silicon in a step of forming a transistor including a high dielectric material and a metal gate (refer to Comparative Example 2-8).
As a technique for removing chlorine while suppressing etching of aluminum, there has been proposed the aqueous solution containing quaternary ammonium hydroxide, and a sugar or a sugar alcohol (Patent Document 8). However, the technique described in Patent Document 8 relates to the method of preventing etching of aluminum from the viewpoint of removal of chlorine, and therefore fails to specify a silicon etching capability of the alkaline stripping solution. More specifically, the technique described in Patent Document 8 is concerned with a technical concept which is quite different from that of the present invention which aims at etching silicon without etching an aluminum film. Further, the aqueous solution described in Patent Document 8 which has an excessively low silicon etch rate is not usable in etching of silicon in the step of forming a transistor including a high dielectric material and a metal gate as aimed by the present invention (refer to Comparative Examples 2-9).
There has also been proposed the stripping solution which is capable of suppressing etching of aluminum and reducing an adhesion strength of an adhesive film (Patent Document 9). However, in Patent Document 9, etching of aluminum with the alkaline stripping solution proposed therein is prevented from such a viewpoint that its capability of reducing an adhesion strength of the adhesive film is not inhibited. Thus, Patent Document 9 fails to describe a silicon etching capability of the alkaline stripping solution. Therefore, the technique described in Patent Document 9 is different from that of the present invention which aims at etching silicon without etching an aluminum film. Further, in Patent Document 9, it is described that the stripping solution used therein is not particularly limited as long as it is in the form of an alkaline solution. However, alkaline compounds capable of etching silicon are limited to specific compounds. Thus, the compounds suitably used in the present invention are not easily suggested from the descriptions of the Patent Document 9 (refer to Comparative Examples 2-10).
As a technique of suppressing etching of aluminum and removing a polyimide orientation film, there has been proposed the aqueous solution containing quaternary ammonium hydroxide, trialkylamine, and an alcohol or an alkyl ether (Patent Document 10). However, the cleaning solution described in Patent Document 10 has a low silicon etching capability and therefore is unsuitable for achieving the object as aimed by the present invention (refer to Comparative Examples 2 to 11).
In consequence, it has been intensively demanded to provide a process for producing a transistor including a laminate formed by laminating at least a high dielectric material film and an aluminum metal gate which is capable of selectively removing a natural silicon oxide film without etching aluminum, an interlayer insulating film and a side wall, and further which is capable of selectively removing silicon without etching aluminum, an interlayer insulating film and a side wall as well as the high dielectric material film.